REVIEWS Radiation enteropathy—pathogenesis, treatment and prevention Martin Hauer-Jensen, James W. Denham and H. Jervoise N. Andreyev Abstract | Changes in cancer incidence and mortality have been modest during the past several decades, but the number of cancer survivors has almost tripled during the same period. With an increasing cohort of cancer survivors, efforts to prevent, diagnose and manage adverse effects of cancer therapy, in general, and those of radiation therapy specifically, have intensified. Many cancer survivors have undergone radiation therapy of tumours in the pelvis or abdomen, thus rendering the bowel at risk of injury. In fact, the current prevalence of patients who have long-term radiation-induced intestinal adverse effects exceeds that of IBD. Considerable progress towards reducing toxicity of radiation therapy has been made by the introduction of so-called dosesculpting treatment techniques, which enable precise delivery of the radiation beam. Moreover, new insights into the underlying pathophysiology have resulted in an improved understanding of mechanisms of radiationinduced bowel toxicity and in development of new diagnostic strategies and management opportunities. This Review discusses the pathogenesis of early and delayed radiation-induced bowel toxicity, presents current management options and outlines priorities for future research. By adding insight into molecular and cellular mechanisms of related bowel disorders, gastroenterologists can substantially strengthen these efforts. Hauer-Jensen, M. et al. Nat. Rev. Gastroenterol. Hepatol. advance online publication 1 April 2014; doi:10.1038/nrgastro.2014.46

Introduction

Surgical Service, Central Arkansas Veterans Healthcare System and Division of Radiation Health, University of Arkansas for Medical Sciences, 4301 West Markham, Little Rock, AR 72205, USA (M.H.‑J.). Department of Radiation Oncology, University of Newcastle, Newcastle, NSW 2308, Australia (J.W.D.). Department of Medicine, Royal Marsden Hospital, Fulham Road, London SW3 6JJ, UK (H.J.N.A.). Correspondence to: M.H.‑J. mhjensen@ life.uams.edu

Radiation therapy is used in at least 50% of patients with cancer and has a crucial role in 25% of cancer cures.1 Despite advances in treatment delivery techniques, radiation toxicity to healthy tissue remains the overwhelmingly most important barrier to cancer cure in patients with localized disease. During radiation therapy of tumours in the abdomen or pelvis, the intestine is at risk of damage. Early radiation enteropathy (intestinal radiation toxicity) occurs within 3 months of radiation therapy and affects the quality of life at the time of treatment. As treatment interruption or changes in the original treatment plan might be required, the likelihood of tumour control is compromized. Delayed radiation enteropathy is a major issue for long-term cancer survivors; this progressive condition has few therapeutic options available and can lead to substantial long-term morbidity and mortality. This Review discusses radiation enteropathy as a clinical problem, as well as the pathological features of the condition and its underlying cellular and molecular mechanisms. Contemporary approaches for prevention and management of radiation enteropathy are also presented. The aim of this Review is to provide an introduction to the subject, tailored to the needs of the gastroenterologist.

Magnitude of the clinical problem Developments in treatment techniques have made it possible to deliver radiation to tumours with much greater Competing interests The authors declare no competing interests.

precision than before. Nevertheless, healthy tissue toxicity remains the single most important radiation-doselimiting factor and obstacle to cancer cure. Moreover, some authors have expressed concern about new treatment techniques and what they mean for the spectrum of toxicities.2 During radiation therapy of tumours in the abdominal cavity or pelvis, parts of the small bowel, colon or rectum are inevitably included in the treatment field and represent important healthy tissues at risk. The incidence and severity of radiation enteropathy are dependent on a number of factors. Therapy-related factors include radiation dose, volume (length) of bowel that is irradiated, time-dose-fractionation parameters and the use of concomitant chemotherapy or biotherapy. Patient-related factors include body mass index (obesity protects, whereas reduced body mass index predisposes to radiation toxicity 3,4); previous abdominal surgery, which increases the risk of radiation-induced bowel injury (peritoneal adhesions lead to fixation of small bowel loops in the radiation field); and certain comorbidities including IBD,5 diabetes,6 vascular disorders7 and collagen vascular disease.8,9 Tobacco smoking is a strong independent predictor of intestinal complications after radiation therapy of tumours in the pelvis or abdomen. Genetic pre­ disposition also has an important role and might explain why certain patients go through therapy without adverse effects, and others, who get exactly the same treatment, experience severe toxicities.10 Radiation enteropathy is generally classified as early (acute) when it occurs within 3 months of radiation therapy, or delayed (chronic) when it occurs >3 months

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REVIEWS Key points ■■ Radiation therapy planning and delivery methods have improved substantially, but the risk of intestinal radiation injury remains the single most important dose-limiting factor in radiation therapy for abdominal and pelvic tumours ■■ Early (acute) radiation enteropathy generally occurs during the course of radiation therapy, whereas delayed (chronic) radiation enteropathy develops after a latency period of variable length ■■ Delayed radiation enteropathy is among the most common radiation-therapyrelated adverse effects; the prevalence of radiation enteropathy exceeds that of IBD ■■ The risk of radiation enteropathy limits the uncomplicated cancer cure rate and adversely affects the quality of life of cancer survivors ■■ As the number of cancer survivors steadily increases, radiation enteropathy represents a significant challenge for future research ■■ Finding safe and effective pharmacological methods to reduce the incidence and severity of radiation enteropathy is an unmet need

after radiation therapy. Annually, it is estimated that >200,000 patients in the USA receive pelvic or abdominal radiation therapy with a 60–80% incidence of symptoms of acute bowel toxicity. Currently, there are >13 million cancer survivors in the USA, and this number will probably increase to 18 million by 2022.11 More than half of these patients are survivors of abdominal or pelvic cancers (Figure 1).12 The incidence of severe (grade 3–4) delayed radiation enteropathy has diminished over time, largely thanks to developments in radiation treatment planning and radiation delivery techniques. However, series with careful follow-up show that at least half of the patients will have some form of chronic gastrointestinal dysfunction. Most clinical studies greatly underestimate the true prevalence of delayed bowel toxicity.13 However, some authors claim that some degree of gastrointestinal dysfunction is an almost inevitable consequence of pelvic or abdominal radiation therapy.14,15 Our conservative estimate of the number of patients with post-radiation intestinal dysfunction living in the USA most certainly a

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b Survivors (13 million) Projected in 2022 (18 million) New cases (1.66 million) Deaths (580,000)

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2

Other abdominal/ pelvic tumours (32%) Breast cancer (22%)

19

75 19 80 19 85 19 90 19 95 20 00 20 05 20 10 20 15 20 20 20 25

0 Year

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Figure 1 | Cancer survivors and cancer prevalence rates in the USA. a | Cancer incidence and death rates have been fairly flat during the past four decades, whereas the cohort of cancer survivors has increased by 2–3% per year, exceeding 13 million people in 2013, and is expected to reach 18 million in 2022.11 Approximately half of all cancer survivors have had abdominal or pelvic tumours,12 many of whom have had or will have radiation therapy. Radiation enteropathy, therefore, seems to be a major obstacle to uncomplicated cancer cures. b | Prevalence rates.

exceeds 1.6 million, which is in contrast to a prevalence of 396 per 100,000 persons for IBD 16 or ~1.4 million people with IBD in the USA.17

Clinical and pathological characteristics Early intestinal injury manifests within days of beginning a course of radiation therapy and is primarily a result of cell death in the rapidly proliferating crypt epithelium and a protracted acute inflammatory reaction in the lamina propria. Crypt cell death results in insufficient replacement of the villus epithelium, breakdown of the mucosal barrier and mucosal inflammation. Figure 2 shows an example of experimental radiation mucositis using a clinically relevant model for localized irradiation of rat small bowel18 and Figure 3 displays an example of clinical radiation mucositis in the rectum from a patient undergoing radiation therapy for prostate cancer. Symptoms of early bowel toxicity (nausea, abdominal pain, diarrhoea and fatigue) develop in 60–80% of patients during radiation therapy of tumours in the abdomen or pelvis. Nausea typically occurs relatively early, whereas diarrhoea and abdominal pain usually become problematic 2–3 weeks into the course of radiation therapy. In most patients, acute symptoms of bowel toxicity resolve within 1–3 months of c­ompleting treatment. Symptoms of delayed bowel toxicity can develop before symptoms attributable to early toxicity subside, but typically present after a latency period of 6 months to 3 years. However, latency periods of 20–30 years after radiation therapy are not uncommon. The pathogenesis of delayed radiation enteropathy is complex and involves changes in most compartments of the intestinal wall. Atrophy of the mucosa, fibrosis of the intestinal wall and microvascular sclerosis are prominent, and are currently irreversible features (Figure 4). The main clinical features of delayed radiation enteropathy are altered intestinal transit, nutrient malabsorption and gut dysmotility.19 Delayed radiation enteropathy is a chronic, often progressive disorder and associated with substantial long-term morbidity. Severe (grade 3–4) late effects, as mentioned above, have become less common than they were in the past. Nevertheless, for example, the incidence of severe toxicity after chemoradiation therapy of cervical cancer remains at ~10%.20 Severe delayed radiation enteropathy might progress to intestinal obstruction, fistulae formation or frank intestinal perforation. Corrective surgery is associated with high postoperative morbidity and mortality. In the long term, the majority of patients have persistent or recurrent symptoms, and ~10% of patients die as a direct result of radiation enteropathy.21,22 Patients with isolated colonic injury have fewer problems with nutrition, fluid and electrolyte balance and their long-term prognosis is better. A comprehensive description of clinical and pathological features of radiation e­nteropathy has been provided elsewhere.23–25

Understanding the toxicity The classic understanding of radiation enteropathy was based entirely on the ‘target cell’ theory, which was formulated in the 1920s and 1930s and provided us with

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Figure 2 | Radiation mucositis in the rat intestine. Unirradiated (control) rat with intestine transposed to the left scrotum (a–c). a | Bioluminescence image (luminol). No increase in bioluminescence indicating little or no myeloperoxidase activity. b | Small intestine stained with anti-transferrin antibody. Few granulocytes are seen in the mucosa or submucosa. c | Small intestine stained with antiED2 (CD163) antibody. There are few macrophages in the mucosa/submucosa. Rat with intestine transposed to the left scrotum and exposed to localized, fractionated irradiation (d–f). d | Bioluminescence image (luminol) 5 days after localized irradiation. Significant increase in bioluminescence indicating substantial myeloperoxidase activity. Intestine procured 2 weeks after irradiation showing e | accumulation of granulocytes in mucosa/submucosa and f | accumulation of macrophages in mucosa or submucosa. Original magnification of all rat intestine images 40×.

a

b

Figure 3 | Human endoscopic biopsy samples of rectal mucosa obtained from a patient before and during ongoing radiation therapy of prostate cancer. a | PAS staining of healthy rectal mucosa before the start of radiation therapy. Note intact surface epithelium, straight glands and the many PAS-positive goblet cells. b | Glandular atrophy, mucosal inflammation, and loss of PAS-positive goblet cells 2 weeks into the course of radiation therapy. Abbreviation, PAS, para-aminosalicylic acid. Original magnification of both images 20×.

a formal framework in the 1940s.26 According to this concept, the intestine was considered a more or less inert tube, covered on the inside by a rapidly proliferating epithelium, with the rest of the bowel tissues more or less irrelevant. The severity of epithelial injury was the only determinant of early pathology, whereas a different, more slowly proliferating, target cell (fibroblast, endothelial cell) was used to explain delayed effects. The sequence of structural and functional manifestations of radiation enteropathy has not changed, but our understanding of the underlying pathobiology has improved over the years. Hence, the contemporary view is that many tissues and cell types in the gut participate and contribute to injury. For example, the intestinal immune system is the largest in the human body and profoundly influences the development of secondary changes after radiation. The enteric nervous system is the second largest nervous system, with a greater number of neurons than the spinal cord, and strongly regulates radiation enteropathy develop­ment.27 The intestinal microvasculature is also recognized as an important contributor to radiation toxicity of the bowel. Furthermore, 100 trillion bacteria in the gut lumen, 10-fold the number of cells in the human body, profoundly influence radiation enteropathy development.28 In other words, we have progressed beyond the single ‘target cell’ concept and now recognize that, in addition to epithelial injury, the intestinal microvasculature, immune mechanisms, neuro­immune interactions, the gut microbiome, the composition of the intraluminal contents and a host of other factors have important roles (Figure 5).

Consequential late effects The recognition that delayed radiation injury might develop in the wake of severe acute injury was recognized clinically by Bourne and colleagues29 and Peters et al.30 subsequently coined the term consequential late effects. Consequential late effects serve an important purpose by helping to eradicate the old dogma of independence between early and delayed radiation effects; improving our understanding of the pathophysiology and pathogenesis of delayed normal tissue injury; and for interpreting and modelling radiation responses in vivo. However, it has become increasingly clear that the terminology fails to recognize the complexity of radiation effects in multicellular tissues and organs; the co-existence of consequential and primary injury (although the response might be skewed in one direction or the other); and evidence that early effects unrelated to cell death might give rise to subsequent chronic injury. Many pharmacological and genetic models have also shown that the relationship cannot be explained simply by consequential late effects. For example, rats deficient in mast cells, sensory nerve-ablated rats and TGF-β heterozygous rats all show dissociations between early and delayed injury,31–34 that is, they all have exacerbated epithelial injury, but reduced levels of intestinal fibrosis. As these observations fundamentally conflicted with the traditional notion of consequential late tissue injury, a new terminology for classifying healthy tissue radiation

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Fractionated radiation therapy

Figure 4 | Resection specimens of small intestine. a | A healthy intestine and b | intestine from a woman with severe delayed radiation enteropathy; note atrophic mucosa and severe fibrosis in the submucosa and subserosa. Original magnification of both images 0.5×.

responses was proposed in 2001.35 According to this classification, there are three types of effect. First, cytocidal effects, in which radiation causes cell death including clonogenic cell death, mitotic catastrophe and apoptosis. Second, functional effects, in which radiation leads to changes including transcription factor activation and protein modification in the intracellular environment, plasma membrane and extracellular space. Third, secondary effects that occur in response to the initial radiation insult, such as cellular inflammation and release of cytokines and other mediators. It is important to remember that all three types of effect interact and contribute to organ dysfunction.

Activating agents

Intestinal lumen

Most radiation therapy regimens are delivered as fractions of 1.8–2.0 Gy on weekdays for a number of weeks. The radiotherapy response differs from other types of tissue damage in that a burst of free radicals is produced, which not only causes immediate DNA damage, but also alters proteins, lipids, carbohydrates and other complex molecules. When considering healthy tissue radiation responses, it is important to also consider that they occur as a result of multiple repetitive injuries (fractions) rather than as a response to a single insult,36 with each fraction contributing to inflammatory cell recruitment as well as to the accumulation of direct tissue injury. Furthermore, each fraction affects tissue that already exhibits a dynamic spectrum of cellular injury, ongoing repair, inflammation and other pathophysiological responses. Therefore, with repetitive radiation exposure, many cellular and molecular responses will be substantially exacerbated, suppressed or substantially altered when compared with the situation after a single exposure to radiation or traumatic injury. Nowhere is this effect more evident than in the intestine. The number of patients with symptoms of toxicity increases steadily during a 6‑week course of fractionated radiation, as does the toxicity score.37 However, mucosal pathology and functional bowel injury (intestinal permeability assessed with differential urinary excretion of disaccharides and monosaccharides) is actually substantially worse after just 2 weeks irradiation than towards the end of the treatment course.37,38 The fact that the patient has received a threefold higher radiation dose at the end of the radiation therapy course than at 2 weeks points to the remarkable regenerative capacity of the intestine and raises i­nteresting questions about the real cause of the symptoms.

Radiation enteropathy as a model of IBD IFN-γ IL-2 LTα FAS-L LTB4

Mucositis

Adverse remodelling

IL-6 IL-12 IL-18 TNF

ROS

IL-1

PARs

Neutrophil activation, adhesion, migration

Proliferation collagen

IL-8 Thrombin

Vessel lumen

Epithelium

Endothelium Platelets, fibrin

Figure 5 | Involvement of the intestinal immune system and microvascular endothelium in the regulation of acute radiation mucositis and subsequent adverse tissue remodelling (intestinal fibrosis). When the mucosal barrier becomes disrupted (such as after radiation exposure), bacterial products and other activating agents gain access to the intestinal tissues and stimulate a variety  of immune cells to produce cytokines and other proinflammatory and anti‑inflammatory mediators. Moreover, radiation-induced endothelial dysfunction leads to loss of thromboresistance, resulting in thrombin formation, neutrophil recruitment and activation, and stimulation of mesenchymal cells. In addition to the mechanisms depicted here, a host of other mechanisms, for example, those related to mast cells, the enteric nervous system and the gut microbiome, have important roles in the pathogenesis of radiation enteropathy. Abbreviations: FAS‑L, Fas ligand; IFN, interferon; IL, interleukin; LTα, Lymphotoxin alpha; LTB4, Leukotriene-B4 omega-hydroxylase 2; PAR, protease-activated receptor; ROS, reactive oxygen species; TNF, tumour necrosis factor.

Radiation enteropathy could be considered an ideal model of gastrointestinal inflammation for several reasons. First, radiation enteropathy is highly clinically relevant; as pointed out above, the prevalence of r­adiation-induced gastrointestinal dysfunction is higher than that of IBD. Second, animal models and patients have identical pathology and pathophysiology, so the translational value of the observations made in the animal model is clear (Figure 6). Third, animal models of radiation enteropathy have made it possible to easily and precisely adjust the dose of the ‘toxic’ agent (radiation), and so dose–response relationships can be investigated in greater detail than is true for other animal models of IBD. For example, a particularly useful, clinically relevant rat model has been published and used extensively for studies of time-dose-fractionation relationships.39 This model, which can be created by a simple surgical pro­cedure prior to irradiation, enables delivery of fractionated irradiation to a defined loop of small intestine and can be further modified to study intraluminal delivery of radiation response modifiers directly into the irradiated bowel loop.40 Finally, by using the identical causative agent (radiation), radiation enteropathy studies can be directly translated to the human disease

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Figure 6 | Radiation-induced changes in the proximal jejunum from nonhuman primates. Proximal jejunum from a | unirradiated Rhesus macaque and b–d | Rhesus macaque 4, 7 and 12 days after exposure to single-dose irradiation. Note conspicuous disappearance of plicae circulares, crypt irregularity and pronounced villus atrophy after irradiation. Partial recovery of post-irradiation changes is seen at 12 days, with near complete recovery of the epithelium and partial reappearance of plicae circulares. Original magnification of all images 1.4×.

situation. Methodological and mechanistic insight from other types of gastrointestinal injury or disease processes (such as IBD, intestinal ischaemia or enteropathy induced by NSAIDs) applied to radiation enteropathy will probably help advance this important area of research. Nevertheless, as with all animal models, those of radiation enteropathy have limitations, for example, in terms of the applicability of radiosensitivity, repair capacity and differential responses to certain treatments.

Outstanding issues Much discussion has evolved around whether the predominant mode of radiation-induced cell death in the epithelium is by mitotic cell death, apoptosis or by some other mechanism. The role of enterocyte apoptosis has been particularly hotly debated. Crypt cell apoptosis has been well described and, to some extent, taken as a measure of intestinal radiation toxicity.41 However, although p53-deficient mice exhibit greatly diminished crypt cell apoptosis, they are actually sensitized to intestinal radiation injury. 42 Moreover, using conditional Bax-deficient mice on a Bak1-deficient background, Kirsch and co-workers showed that deletion of these proapoptoti­c genes from the intestinal epithelium did not protect the mice from intestinal radiation syndrome.43 It is possible that the explanation is to be found in the effect of p53/p21 on apoptotic versus non-apoptotic cell death.44 Radiation predominantly kills rapidly proliferating cells, such as the progenitor cells in the intestinal crypts, which leads to insufficient replacement of the villus epithelium. Much attention has therefore been devoted to understanding the intestinal stem cell population.45,46 Currently, at least two types of intestinal stem cell are believed to exist, namely LGR5 (leucine-rich

repeat-containing G‑protein coupled receptor  5) stem cells and polycomb complex protein BMI‑1 stem cells.47 In mice, Lgr5-positive cells are normally mito­ tically active and are considered radioresistant, whereas Bmi1-positive cells are quiescent and considered more radio­sensitive.48 Although both types of stem cell seem to contribute to regeneration of intestinal crypts after irradi­ation,49 data from a study published in 2014 indicate that only Lgr5-postive cells are required.50 The intense activity in the field of regenerative medicine, as well as the recognition of the unmet need for medical countermeasures for use in radiological and/or nuclear emergencies, highlights the area of intestinal stem cells as particularly promising. The microvasculature is known to have a central role in the regulation of radiation responses in many healthy tissues, including the intestine.51,52 Radiation induces many changes in endothelial cells, such as apoptosis, detachment from the basement membrane, increased endothelial permeability, interstitial fibrin deposition and shifting of the thrombo-haemorrhagic balance towards coagulation. Although microvascular injury clearly has at least some role in explaining the self-perpetuating nature of chronic radiation fibrosis,53–55 its role in early radiation enteropathy, particularly in the so-called gastro­intestinal acute radiation syndrome, is more controversial. Paris and co-workers, in 2001, published the first in a series of articles showing that endothelial apoptosis was the primary lesion responsible for this syndrome.56 Although subsequent publications are supportive,57 there has been serious challenge to this notion by groups who have been unable to identify a role for endothelial apoptosis.43,58,59 There is clearly a need for additional research in this important area. Moreover, as the radiation threshold for apoptosis in the endothelium is high, endothelial cell apoptosis is unlikely to play a significant part during conventionally fractionated radiation therapy. Despite this (still ongoing) controversy, it is known from other fields of biology that preserving the intestinal micro­circulation after an insult has a protective effect on the gut epithelium and the intestinal mucosa.60 Hence, it is conceivable that radiation-induced endothelial cell apoptosis might be the bellwether or tip of the iceberg, indicating a broader state of dysfunction in the intestinal microvasculature, and that endothelial dysfunction indirectly affects the radiation tolerance and/or repair capacity of the crypt epithelium.61,62 Under healthy conditions, the enteric nervous system regulates intestinal motility, blood flow and enterocyte function, and has a central role in maintaining the physiological state of the intestinal mucosa, as well as in co­ordinating inflammatory and fibroproliferative processes. Interactions between afferent nerves, mast cells and other cells of the resident mucosal immune system maintain mucosal homeostasis and ensure an appropriate response to injury, including radiation entero­pathy develop­m ent. 27 These interactions are mediated by substance P, calcitonin gene-related peptide and other neuropeptides secreted by the sensory nerves, whereas resident immune cells signal to enteric nerves by the

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REVIEWS release of cytokines, growth factors and other mediators. Elucidating the role of interactions between the enteric nerves and the local immune system in radiation entero­ pathy development will probably identify new targets for intervention and provide additional mechanistic insight. The parallel between radiation-induced inflammation and primary IBD is noteworthy: colonization with microbiota is necessary for the development of colitis in mouse models, whereas germ-free mice are resistant to both inflammatory colitis and radiation enteropathy development.63 A similar parallel is apparent for NSAIDinduced enteropathy, which is exacerbated by dysbiosis and greatly attenuated in germ-free animals. 64 Owing to the critical role of gut-associated sepsis in lethality after exposure to total body irradiation, substantial literature exists, dating back >5 decades, on the importance of intestinal bacteria in the radiation response.65–67 Most intestinal bacteria are considered detrimental in the context of radiation and antibiotic therapy or ‘gutdecontamination’ generally improves outcome in experimental animals after exposure to total body irradiation. Early studies also suggested that microbiota are critical in humans. By contrast, it is clear that sterilizing the bowel is impossible in the clinical situation, antibiotic use tends to select resistant organisms and that certain bacteria are more harmful than others. As with IBD, the potential of probiotics in mitigating gastrointestinal inflam­mation during radiation therapy has generated substantial interest.68–71 Based on encouraging results from several randomized controlled trials with Lactobacillus-based probiotics, the Multinational Association for Supportive Care in Cancer in the 2013 update to their evidencebased guidelines recommended probiotics for prevention of radiotherapy-induced intestinal adverse effects.72 Techniques for assessment of bacterial flora in the healthy state and after exposure to radiation73–76 combined with development of powerful gnotobiotic animal models77,78 and an improved understanding of the importance of the gut microbiome in health and disease are some of the reasons why this promising area of research is constantly evolving.

Therapy and prevention Many natural products, peptides and small molecules have been tested preclinically for the purpose of preventing, mitigating (strategies that are applied after irradiation, but before symptoms occur), and treating radiation enteropathy. However, substantial differences exist between what has proven to be effective in animal models and what has proven to be effective clinically, especially when proper evidence-based criteria are used, as discussed in more detail below.79

Clinical strategies The management of acute radiation enteropathy remains largely symptomatic and follows guidelines for treating similar symptoms in other situations. Patients with severe diarrhoea who do not respond to first-line antidiarrhoeal medication can be treated with octreotide or other somatostatin analogues. The free-radical scavenger

amifostine is the only drug currently approved by the FDA for reduction of radiation therapy adverse effects. Although amifostine has shown impressive effects in some animal studies, and has also shown some effect in preventing clinical gastrointestinal radiation toxicity, serious adverse effects from the drug (nausea, vomiting and hypotension), a narrow therapeutic time window and lingering concerns about the possibility of tumour protection have severely hampered its use.80 Medical management of patients with delayed radiation enteropathy should be individualized and directed at the specific underlying abnormalities. A comprehensive discussion of specific diagnosis and management principles and algorithms in delayed radiation enteropathy is beyond the scope of this Review. However, it is evident that many patients can be markedly helped by a systematic approach.81 An algorithm depicting the approach to patients with radiation-induced bowel problems and common treatment options that is used at the Royal Marsden Hospital, London, UK, is provided in Figure 7. Rectal radiation injury (radiation proctopathy) is usually considered separately from injury to the small bowel and colon and is a common complication of radiation therapy for prostate cancer. First-line therapy for radiation proctopathy with bleeding is sucralfate enemas, which often produce rapid and dramatic effects.82,83 In patients with haemorrhagic proctopathy that is refractory to sucralfate enemas, bleeding can be controlled with local (endoscopic) interventions or hyperbaric oxygen therapy.84 Evidence-based reviews of strategies to minimize early and/or delayed radiation enteropathy and radiation proctopathy have been published, for example, by the Multinational Association of Supportive Care in Cancer.79

Preclinical research Drugs to protect healthy tissues from radiation represent a striking unmet need, both in cancer treatment and in the context of radiological emergencies. Therefore, there is intense interest in finding safe, non-toxic ra­dioprotective compounds that do not confer tumour protection. Interventions aimed at protecting healthy tissue against radiation injury fall into two conceptually different categories. The first involves strategies that interfere directly with radiation-specific mechanisms of injury. Most radiation injury is initiated by reactive oxygen species, therefore, antioxidants, free radical scavengers and various cytoprotectors have been the subjects of active study for more than half a century. For example, amifostine is a potent scavenger of free radicals.85 Superoxide dismutase and various superoxide dismutase mimetics are also being pursued as potential radioprotective strategies.86 The vitamin E analogue γ-tocotrienol is the most potent non-toxic natural radioprotective compound discovered to date.87 The problems with most of these compounds in the cancer therapy situation, however, is that it is often unclear to what extent they also protect tumour cells. Toxicity and a narrow therapeutic time window are also obstacles related to some of the compounds in this category.

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REVIEWS Refer appropriately e.g.: ■ Gynaecology ■ Urology ■ Psychological support ■ Pain team ■ Rheumatology ■ Lymphoedema services ■ Physiotherapy

Bleeding from radiation-induced telangiectasia

Non-GI unmet needs identified

GI symptoms

Holistic assessment

Recurrence unlikely

Minimal symptoms Not affecting QOL No alarm features

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Positive

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Reassure ■ Flexible endoscopy assessment ■ Optimize bowel function ■ Bleeding not affecting QOL: reassure; no treatment needed ■ Bleeding affecting QOL; stop/reduce anticoagulants if possible; if severe start sucralfate enemas ■ Discuss definitive treatment options: ■ hyperbaric oxygen therapy (may improve other symptoms but timeconsuming) ■ argon plasma coagulation (easily available but high risk of serious complications) ■ formalin therapy (simple to perform but long-term outcomes unknown)

Are there lifestyle/ medication/ dietary/ endocrine/ psychological/ other non-GI causes for the symptoms?

Intermittent or constant steatorrhea or diarrhoea Exclude: SIBO/BAM/ pancreatic insufficiency/ FFA malabsorption/ lactose intolerance/ endocrine causes

Nocturnal defecation/ obstructive episodes indicate organic disease

Urgency/faecal incontinence If no organic disease: ■ Assess fibre intake ■ Pelvic floor and toileting exercises ■ Stool bulking agent ■ Anti-diarrhoeals ■ Tricyclic antidepressants ■ Biofeedback

Figure 7 | Algorithm depicting simplified principles of work-up and common approaches for managing patients with delayed gastrointestinal symptoms after radiation therapy used at the Royal Marsden Hospital, London, UK. Abbreviations: BAM, bile acid malabsorption; FFA, free fatty acid; GI, gastrointestinal; QOL, quality of life; SIBO, small intestinal bacterial overgrowth.

The second, fundamentally different approach consists of agents that modulate various pathophysiological, cellular or molecular responses that occur downstream from radiation. These interventions seek to increase radiation tolerance, ameliorate secondary normal tissue injury or enhance repair capacity. Such approaches include, for example, various immune-modulating drugs,40,88 entero­ trophic agents,89–96 compounds that modulate intra­ luminal contents,97,98 and a variety of other strategies.99–104 Interventions that target downstream radiation effects might be generally more appealing in the cancer treatment situation because they do not interfere directly with the mechanism of radiation. Therefore, tumour protection is often, albeit by no means always, less of a concern than it is with free radical scavengers and antioxidants. A comprehensive, up-to-date discussion of the various strategies that have been and are under investigation is beyond the scope of this Review, but has been covered elsewhere.25

Screening for compounds Many compounds have demonstrated fairly robust radioprotective effects in animal studies. However, few have advanced to clinical testing and most of those that do fail, either because of clinical toxicities, a lacklustre protective effect or concerns about tumour protection. Moreover, substantial barriers need to be overcome in the drug development process. First, there is a false perception that the prevalence of radiation enteropathy is lower than it

really is (again, the prevalence of radiation enteropathy is actually higher than that of IBD). Second, there is a lack of general public appeal for radiation enteropathy and also a lack of interest from clinicians and institutions (sometimes motivated by financial or medico-legal considerations). Third, radiation enteropathy is a complex disorder that requires multidisciplinary expertise often not readily available in the cancer treatment environment. Fourth, many clinicians feel that any treatment of delayed radiation enteropathy is unlikely to be successful. Finally, the pharmaceutical and biotechnology industry has devoted little attention to radiation enteropathy research from. The interest in finding so-called medical counter­ measures against radiation (drugs for use in the radiological and nuclear emergency situation, in which radiation injury to the bone marrow and intestine is the main determinant of survival) has spawned a resurgence in activities to find compounds to protect the intestine against radiation. Such drugs can potentially also benefit the cancer patient who undergoes radiation therapy—so called dual benefit drugs. The recommended steps in the development process of such drugs have been reviewed by Movsas and co-workers.105 First, for a candidate drug to be selected, there needs to be evidence of general or organ-specific healthy tissue protection and lack of tumour protection. Drug candidates that fulfil these criteria undergo testing to determine the maximal-tolerated dose, pharmacokinetics, pharmacodynamics and toxicity.

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REVIEWS Box 1 | Priorities for future research ■■ Obtain an improved understanding of physiological versus pathological responses of the intestine to radiation injury ■■ Perform clinical, epidemiological and outcomes studies in well-defined cohorts of cancer survivors to define true prevalence of late effects of radiation ■■ Determine the medical, quality-of-life-related, social and financial consequences of radiation-induced bowel injury ■■ Develop predictive assays to identify patients who are more prone than others to develop delayed healthy tissue toxicity after radiation therapy ■■ Strengthen molecular epidemiology research aimed at identifying genetic or epigenetic characteristics that correlate with susceptibility to delayed radiation enteropathy ■■ Testing of radiation response modifiers in clinical trials ■■ Engage the pharmaceutical and biotechnology industries in developing strategies to modulate radiation enteropathy

The next stage is to verify evidence of normal tissue protection in vitro and in vivo, absence of tumour protection in vivo in pertinent xenograft models and further testing of biological mechanisms. Finally, before proceeding to clinical screening, comprehensive drug evaluation and formulation studies should be performed.

The role of the gastroenterologist Few gastroenterologists fully appreciate how much can be achieved for the symptomatic patient after pelvic i­rradiation. Moreover, they also do not recognize the value of a preclinical model, which has so many parallels with IBD and where the initiating insult (radiation) can be so finely adjusted. Indeed, although fibrosis in the liver has deservedly received substantial attention, the mechanisms of intestinal fibrosis have barely been investigated, even though it is a critical pathophysiological process in a large numbers of patients after radiotherapy. Progressive fibrosis not only contributes to substantial morbidity, but it is also important in many other gastrointestinal diseases, such as pouchitis, Crohn’s disease, ischaemic colitis and scleroderma. Moreover, progressive fibrosis is easy to study in a model in which the onset of fibrosis can be predicted. Conversely, the radiation biology community has, until now, been largely deprived of the insights which gastroenterologists could bring from their 1.

2.

3.

4.

5.

DeVita, V. T., Lawrence, T. S. & Rosenberg, S. A. DeVita, Hellman, and Rosenberg’s Cancer. Principles & Practice of Oncology (Wolters Kluwer Health/Lippincott Williams & Wilkins, 2011). Goitein, M. How best to dispose of extra-tumoral dose: a cautionary note for intensity-modulated radiation therapy. Int. J. Radiat. Oncol. Biol. Phys. 75, 1–3 (2009). Eifel, P. J., Jhingran, A., Bodurka, D. C., Levenback, C. & Thames, H. Correlation of smoking history and other patient characteristics with major complications of pelvic radiation therapy for cervical cancer. J. Clin. Oncol. 20, 3651–3657 (2002). Wedlake, L. J. et al. Predicting late effects of pelvic radiotherapy: is there a better approach? Int. J. Radiat. Oncol. Biol. Phys. 78, 1163–1170 (2010). Willett, C. G. et al. Acute and late toxicity of patients with inflammatory bowel disease undergoing irradiation for abdominal and pelvic

knowledge of gastrointestinal pathophysiology in other disease processes. Cross-fertilization between the fields of gastroenterology and radiation biology might thus generate substantial methodological and mechanistic insight into gastrointestinal disease processes initiated by radiation, as well as how these processes are influenced by human genetic profiles, immunological responses and microbial make-up.

Conclusions Despite technological advances in radiation therapy, radiation enteropathy remains an important obstacle to uncomplicated cancer cures and is a much more important clinical problem than previously recognized—the prevalence of radiation enteropathy exceeds that of IBD. The pathogenesis of radiation enteropathy is multi­ factorial and far more complex than previously assumed and the traditional ‘target cell’ concept is largely obsolete. Although some correlation exists, histopathological and endoscopic changes do not parallel subjective symptoms. The complexity of this condition requires rethinking of some of the old dogmas, but opens up the field for development of exciting new therapeutic strategies. As a model of IBD, radiation enteropathy offers substantial advantages in terms of methods and clinical relevance. A paradigm shift is needed to make it possible to adequately deal with the ever-increasing cohort of cancer survivors and their adverse effects (Box 1). Review criteria Comprehensive searches in Ovid and/or PubMed were performed by combining the following search statements with “AND”: Statement #1: exp abnormalities, radiationinduced/ or exp dose-response relationship, radiation/ or exp radiation/ or exp radiation dosage/ or exp radiation effects/ or exp radiation injuries/ or exp radiation injuries, experimental/ or exp radiation oncology/ or exp radiation pneumonitis/ or exp radiation tolerance/ or exp radiation, ionizing/ or radiation.mp. Statement #2: exp fibrosis/ or exp inflammation/ or exp inflammation mediators/. Statement #3: exp intestines/ or intestine. mp. or exp intestinal diseases/ or intestinal diseases.mp

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